1. Field of the Invention
Aspects of the present invention relate to a laser induced thermal imaging method, a method of patterning an organic layer using the same, and a method of fabricating an organic light emitting diode (OLED) display device using the same, and more particularly, to a method of patterning an organic layer using a reverse transfer phenomenon in a laser induced thermal imaging method.
2. Description of the Related Art
Today, the demand of a user to rapidly obtain exact information in one's hand is growing due to the advent of the high information age, and therefore, the development of display devices, which are lightweight and slim which can be easily held, and have a fast data processing speed, is rapidly progressing. Among the display devices, an organic light emitting diode (OLED) display device is a self-emitting device, which emits light by recombination of an electron with a hole in an organic light emitting layer when a voltage is applied to an organic layer including the organic emitting layer. For this reason, the OLED display device does not need a back light, unlike an LCD device, and thus can be manufactured to have a lightweight and slim body and have a simple process. In addition, the OLED display device has the same level of response speed as a cathode ray tube (CRT), and low voltage driving, high emission efficiency and a wide viewing angle. For this reason, attention to the OLED display device is increased sharply as a next generation display.
Here, according to the material for the organic layer, particularly, the organic emitting layer, the OLED display devices are classified into a small molecule OLED display device and a polymer OLED display device.
The small molecule OLED display device includes multiple organic layers having different functions, including a hole injection layer, a hole transport layer, an emitting layer, a hole blocking layer, and an electron injection layer, which are disposed between an anode and a cathode. Here, the layers can be controlled by being doped or replaced with a material having an appropriate energy level to prevent accumulation of charges. However, since the small molecule organic layer is formed by vacuum deposition, it is difficult to be used to implement a large-sized display.
On the other hand, the polymer OLED display device may be manufactured in a single-layer structure including an organic emitting layer between an anode and a cathode, or in a double-layer structure including a hole transport layer, so that the device becomes thinner. The organic layer is formed by wet coating at atmospheric pressure. As a result, a production cost can be reduced, and a large-sized OLED display device can be easily manufactured.
Here, a single-color device may be simply manufactured using a polymer OLED display device by spin coating, but it has a lower efficiency and a shorter lifespan than that manufactured using a small molecule OLED display device. A full-color device may be implemented by patterning emitting layers emitting the three primary colors such as R, G, and B to the OLED display device. Here, an organic layer of the small molecule OLED display device may be patterned through deposition using a shadow mask, and an organic layer of the polymer OLED display device may be patterned through inkjet printing or laser induced thermal imaging (LITI). Among them, the LITI method may utilize the characteristics of the spin coating method, so that when it is applied to a large-sized device, the device can obtain excellent uniformity in pixels. In addition, since the LITI method is a dry process, not a wet process, it can solve the problem of a decreasing lifespan due to a solvent, and achieve a fine pattern of the organic layer.
To apply the LITI method, basically, a light source, and organic light emitting diode substrates such as a substrate and a donor substrate are needed. The donor substrate is composed of a base layer, a light-to-heat conversion layer, and a transfer layer. According to the LITI method, light emitted from the light source is absorbed to the light-to-heat conversion layer of the donor substrate, and converted into thermal energy. Thereby, an organic material formed on the transfer layer is transferred to the substrate.
FIGS. 1A and 1B are cross-sectional views illustrating a conventional method of manufacturing an organic light emitting diode display device through a LITI method.
Referring to FIG. 1A, an insulating substrate 101 is prepared, and a first electrode 102 is formed on the insulating substrate by patterning.
Subsequently, a pixel defining layer 103 defining R, G and B pixel regions is formed, and thus a substrate 100 is completed.
Meanwhile, a light-to-heat conversion layer 32 and a transfer layer 33 are sequentially stacked on a base layer 31, and thus a donor substrate 30 is completed.
Afterwards, the pixel region of the substrate 100 is aligned to face the transfer layer of the donor substrate 30, and laser is irradiated to a region of the base layer 31 of the donor substrate to be transferred.
Subsequently, referring to FIG. 1B, after an organic light-emitting material is transferred to the pixel region of the substrate 100, the donor substrate 30 is removed, thereby forming an organic emitting layer pattern 33′.
Afterwards, as shown in FIG. 1C, a second electrode 104 is formed on the organic layer, and thus an OLED display device can be completed.
Consequently, according to the conventional art, to manufacture an OLED display device through a LITI method, laser is irradiated to the base layer in the region to be transferred to transfer the organic light-emitting material to the pixel region of the substrate.
However, due to the size (width) limit of a laser beam, the size of a pattern which can be transferred by a cycle of a transfer process is limited, and thus stitching mura occurs between widths of the laser beams.
FIG. 2 is a photograph showing stitching mura occurring in the conventional OLED display device manufactured by the LITI method, and referring to FIG. 2, due to the size limit of the laser beam, it can be seen that the stitching mura occurs between the widths of the laser beams.